: LexA repressor of E. coli plays a central role in regulation of the SOS response to DNA damage. LexA represses a set of about 20 genes during normal growth. After DNA damage, RecA protein is activated to a form that mediates cleavage of LexA. Cleavage inactivates LexA, leading to derepression of the SOS regulon. The basis for the proposed work is the crystal structures of several mutant LexA proteins. Two forms of LexA are observed. In the NC form, the cleavage site is distant from the active site. In the C form, the cleavage site lies in the active site and is adjacent to the nucleophile that attacks the peptide bond. It is proposed to use the structures as a guide for further analysis of several LexA functions. First, the model will be tested that these forms represent the mechanism controlling reactivity of LexA. Mutants will be made that are predicted to stabilize the C form, and their cleavage rates will be tested. Interesting proteins will be analyzed by x-ray crystallography. Second, dimerization mutants will be made and characterized. Third, in order to characterize further the chemical mechanism of LexA cleavage, efforts will be made to develop a simple substrate for this reaction, and to isolate a covalent intermediate. Fourth, the model will be tested that other cleavable proteins in the LexA superfamily, notably UmuD and lambda repressor, have the same mechanism for controlling their cleavage reactions. Fifth, the structure will be used as a guide to identify portions of LexA involved in interaction with activated RecA. RecA-mediated cleavage will be analyzed in more detail. Efforts will be made to solve the structure of a RecA:LexA complex.